The objective of this project is (1) to study the effects of Epstein-Barr virus (EBV) infections, whether accompanied by signs of infectious mononucleosis or silent, on the course of pregnancy and the development of the offspring and (2) to determine whether pregnancy may activate latent, persistent EBV infections and whether such a reactivation may have consequences for the fetus. Although herpes simplex, varicella zoster, and cytomegalovirus have been associated with disease in the fetus and the newborn only preliminary data is available for EBV. The study population will consist of pregnant women (3600) and infants born to them during the course of the study. Sera obtained for rubella antibody titers will be screened for EBV-specific antibodies. Positive sera will be titrated for IgG and, when indicated, IgM antibodies to viral capsid antigen (VCA) and for IgG antibodies to the D (diffuse) and R (restricted) components of the early antigen complex in indirect immunofluorescence tests, and for antibodies to the EBV-associated nuclear antigen (EBNA) in anti-complement immunofluorescence tests. The women will be divided into those who are susceptible and recent seroconverters (Group 1) and those with antibody evidence of past infection (Group 2). Subsequent sera from susceptible women will be immediately tested for the appearance of EBV-specific antibodies. Infants born to mothers who are seroconverters will be followed clinically and serologically. Sera from women in Group 2 will be analyzed for evidence of reactivation of the latent, persistent viral carrier state.